Radiation Fog

How Radiation Fog Forms

Radiation fog develops exclusively over land, typically during the night or early morning. The process begins when the ground rapidly loses heat through radiation under clear skies. As the ground cools, it chills the air in direct contact with it. If this air is moist and the temperature drops to the dew point, condensation occurs, forming fog. Key conditions for radiation fog include:

• Clear or nearly clear skies (to allow maximum heat loss)
• Light winds (about 2–8 knots)
• High relative humidity
• Land surfaces (not water)

If winds are too calm, dew forms instead of fog; if winds are too strong, the fog may be mixed away or lifted into low stratus clouds.

Characteristics and Vertical Extent

Radiation fog is usually shallow, with an average vertical extent of less than 500 ft. It tends to form in low-lying areas such as valleys, where cold air can accumulate overnight. The fog can be patchy, often affecting some parts of an airfield while leaving others clear. Industrial areas may see more frequent fog due to the presence of condensation nuclei.

Dissipation of Radiation Fog

Radiation fog typically dissipates after sunrise as the sun heats the ground, warming the air and causing the fog to evaporate. In winter, weak sunlight may not be enough to clear the fog, allowing it to persist throughout the day or even for several days. Increasing wind speeds can also lift and disperse the fog, often converting it into a low cloud layer before it clears completely.

Operational Relevance

Radiation fog can form quickly and unpredictably, making it a significant hazard for aviation, particularly during early morning operations. Pilots and meteorologists closely monitor temperature-dew point spreads and local topography to anticipate fog formation and persistence.